The complement system is a crucial component of the innate immunity system and comprises a group of proteins that are normally present in an inactive state. These proteins are organized in three activation pathways: the classical, the lectin, and the alternative pathways (V. M. Holers, In Clinical Immunology: Principles and Practice, ed. R. R. Rich, Mosby Press; 1996, 363-391). Molecules from microorganisms, antibodies or cellular components can activate these pathways resulting in the formation of protease complexes known as the C3-convertase and the C5-convertase. The classical pathway is a calcium/magnesium-dependent cascade, which is normally activated by the formation of antigen-antibody complexes. It can also be activated in an antibody-independent manner by the binding of C-reactive protein complexed to ligand and by many pathogens including gram-negative bacteria. The alternative pathway is a magnesium-dependent cascade which is activated by deposition and activation of C3 on certain susceptible surfaces (e.g., cell wall polysaccharides of yeast and bacteria, and certain biopolymer materials).
Factor B may be a suitable target for the inhibition of this amplification of the complement pathways because its plasma concentration in humans is typically about 200 μg/mL (or about 2 μM), and it has been shown to be a critical enzyme for activation of the alternative complement pathway (P. H. Lesavre and H. J. Müller-Eberhard. J. Exp. Med., 1978; 148: 1498-1510; J. E. Volanakis et al., New Eng. J. Med., 1985; 312:395-401).
Macular degeneration is a clinical term that is used to describe a family of diseases that are characterized by a progressive loss of central vision associated with abnormalities of Bruch's membrane, the choroid, the neural retina and/or the retinal pigment epithelium. In the center of the retina is the macula lutea, which is about ⅓ to ½ cm in diameter. The macula provides detailed vision, particularly in the center (the fovea), because the cones are higher in density and because of the high ratio of ganglion cells to photoreceptor cells. Blood vessels, ganglion cells, inner nuclear layer and cells, and the plexiform layers are all displaced to the side (rather than resting above the photoreceptor cells), thereby allowing light a more direct path to the cones. Under the retina is the choroid, a part of the uveal tract, and the retinal pigmented epithelium (RPE), which is between the neural retina and the choroid. The choroidal blood vessels provide nutrition to the retina and its visual cells.
Age-related macular degeneration (AMD), the most prevalent form of macular degeneration, is associated with progressive loss of visual acuity in the central portion of the visual field, changes in color vision, and abnormal dark adaptation and sensitivity. Two principal clinical manifestations of AMD have been described as the dry, or atrophic, form and the neovascular, or exudative, form. The dry form is associated with atrophic cell death of the central retina or macula, which is required for fine vision used for activities such as reading, driving or recognizing faces. About 10-20% of these AMD patients progress to the second form of AMD, known as neovascular AMD (also referred to as wet AMD).
Neovascular AMD is characterized by the abnormal growth of blood vessels under the macula and vascular leakage, resulting in displacement of the retina, hemorrhage and scarring. This results in a deterioration of sight over a period of weeks to years. Neovascular AMD cases originate from intermediate or advanced dry AMD. The neovascular form accounts for 85% of legal blindness due to AMD. In neovascular AMD, as the abnormal blood vessels leak fluid and blood, scar tissue is formed that destroys the central retina.
The new blood vessels in neovascular AMD are usually derived from the choroid and are referred to as choroidal neovascularizaton (CNV). The pathogenesis of new choroidal vessels is poorly understood, but such factors as inflammation, ischemia, and local production of angiogenic factors are thought to be important. A published study suggests that CNV is caused by complement activation in a mouse laser model (Bora P. S., J. Immunol. 2005; 174; 491-497).
Human genetic evidence implicates the involvement of the complement system, particularly the alternative pathway, in the pathogenesis of Age-related Macular Degeneration (AMD). Significant associations have been found between AMD and polymorphisms in complement factor H (CFH) (Edwards A O, et al. Complement factor H polymorphism and age-related macular degeneration. Science. 2005 Apr. 15; 308(5720):421-4; Hageman G S, et al A common haplotype in the complement regulatory gene factor H (HF1/CFH) predisposes individuals to age-related macular degeneration. Proc Natl Acad Sci USA. 2005 May 17; 102(20):7227-32; Haines J L, et al. Complement factor H variant increases the risk of age-related macular degeneration. Science. 2005 Apr. 15; 308(5720):419-21; Klein R. J, et al Complement factor H polymorphism in age-related macular degeneration. Science. 2005 Apr. 15; 308(5720):385-9; Lau L I, et al. Association of the Y402H polymorphism in complement factor H gene and neovascular age-related macular degeneration in Chinese patients. Invest Ophthalmol Vis Sci. 2006 August; 47(8):3242-6; Simonelli F, et al. Polymorphism p.402Y>H in the complement factor H protein is a risk factor for age related macular degeneration in an Italian population. Br J Ophthalmol. 2006 September; 90(9):1142-5; and Zareparsi S, et al Strong association of the Y402H variant in complement factor H at 1q32 with susceptibility to age-related macular degeneration. Am J Hum Genet. 2005 July; 77(1):149-53.), complement factor B (CFB) and complement C2 (Gold B, et al. Variation in factor B (BF) and complement component 2 (C2) genes is associated with age-related macular degeneration. Nat Genet. 2006 April; 38(4):458-62 and Jakobsdottir J, et al. C2 and CFB genes inage-related maculopathy and joint action with CFH and LOC387715 genes. PLoS One. 2008 May 21; 3(5):e2199), and most recently in complement C3 (Despriet D D, et al Complement component C3 and risk of age-related macular degeneration. Ophthalmology. 2009 March; 116(3):474-480.e2; Maller J B, et al Variation in complement factor 3 is associated with risk of age-related macular degeneration. Nat Genet. 2007 October; 39(10):1200-1 and Park K H, et al Complement component 3 (C3) haplotypes and risk of advanced age-related macular degeneration. Invest Ophthalmol Vis Sci. 2009 July; 50(7):3386-93. Epub 2009 Feb. 21.). Taken together, the genetic variations in the alternative pathway components CFH, CFB, and C3 can predict clinical outcome in nearly 80% of cases.
Currently there is no proven medical therapy for dry AMD and many patients with neovascular AMD become legally blind despite current therapy with anti-VEGF agents such as Lucentis. Thus, it would be desirable to provide therapeutic agents for the treatment or prevention of complement mediated diseases and particularly for the treatment of AMD.